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Greater viewing enjoyment with Krupp VDM: New material developed for flat TV screens

In the coming year, Krupp VDM GmbH's strip business will supply around 1,000 metric tons of Pernifer 42 TVR to a well-known television tube manufacturer. The customer will use the material to make solid frames for the shadow masks in large flat screens. Pernifer 42 TVR is a new material developed specially by Krupp VDM GmbH for the rapidly expanding wide flat screen market and registered for patent. The material displays extremely low thermal expansion, good mechanical strength even at high temperatures and good fabricating properties.

On color TV tubes, the screen - depending on size - is coated with 250,000 to 2.5 million luminous dots (pixels) in the primary colors red, green and blue. The pixels convert the energy from three electron beams, which scan the screen in horizontal lines, into visible light. Picture clarity is provided by a 0.1 to 0.2 mm thick metal mask fitted around 20 millimeters in front of the screen. This mask contains holes matching the number of pixels to direct the electron beams accurately to the corresponding dots. The adjacent pixels are shadowed by the mask material. The shadow mask absorbs a large part of the energy from the electron beams and heats up to around 100°C.

Pernifer 36, a Krupp VDM material which prevents excessive thermal expansion of the shadow mask to ensure constant picture clarity, has been available for some time. At temperatures between 20 and 100°C it displays a coefficient of thermal expansion around ten times lower than that of conventional steels. The bimetal and trimetal springs used to hold shadow masks in place are also made of VDM materials. With Pernifer 42 TVR Krupp VDM has now introduced a new alternative material for the low-cost, reliable manufacture of true-flat screens.

The need for a new material such as Pernifer 42 TVR stems from the fact that an additional component is required to stabilize the shadow masks on true-flat screens. In conventional picture tubes with curved screens, the shadow masks are also curved to enhance their strength. This is not possible on flat screens, so the shadow mask is stretched onto a solid frame to keep it in shape. The frame material must not distort even at temperatures of around 100°C so as to maintain the tension in the shadow mask.

Further requirements relate to a heat treatment process in which the frame and pre-tensioned shadow mask are blackened to enhance picture quality and improve the magnetic properties of the metal to absorb magnetic influences which would otherwise deflect the electron beams and distort the television picture. Minimal thermal expansion of the frame material is vital here to ensure that the shadow mask is not overstretched by the high temperature and remains under tension once it has cooled down again.

To satisfy the heat resistance and expansion requirements for the frame material Krupp VDM GmbH developed Pernifer 42 TVR, a ferro-nickel alloy which contains around 43 weight percent nickel plus precipitation hardening elements such as titanium and niobium. After hardening, the weldable material has a tensile strength in excess of 1,000 Mp. Pernifer 42 TVR displays no creep ex-pansion under stress and its temperature-dependent expansion is adapted to the expansion behavior of the shadow mask material.

Some metallurgical refinement was required because although the alloying elements titanium and niobium ensure the alloy's mechanical strength through precipitation hardening they can also have a negative impact on thermal expansion. It is therefore essential that these elements are precisely proportioned in the melt: the permitted tolerances for titanium and niobium content are measured in hundredths of a percent.

Working directly with the customer, Krupp VDM developed the alloy in just nine months. Trials and simulations in VDM's test lab ran partly in parallel with the rolling of commercially produced melts to strip and the testing of same in the user's manufacturing processes. Since August Pernifer 42 TVR has been used in volume production, with the customer processing several hundred metric tons of the material to date. Other manufacturers from the electronics sector have expressed an interest in the new material.